Nanocomposites are polymers reinforced with nanometer sized particles, i.e., particles with a dimension on the order of 1 to several hundred nanometers, herein referred to as “nanoparticles.”
Polymer-layered silicate nanocomposites incorporate a layered clay mineral filler in a polymer matrix. Layered silicates are made up of several hundred thin platelet layers stacked into an orderly packet known as a tactoid. Each of these platelets is characterized by a large aspect ratio (diameter/thickness on the order of 100-1000). Accordingly, when the clay is dispersed homogeneously and exfoliated as individual platelets throughout the polymer matrix, dramatic increases in strength, flexural and Young's modulus, and heat distortion temperature are observed at very low filler loadings (<10% by weight) because of the large surface area contact between polymer and filler. Two types of clay minerals are commonly used in nanocomposites: kaolin and smectite. The molecules of kaolin are arranged in two sheets or platelets, one of silica and one of alumina. Smectites, such as sodium montmorillonite and calcium montmorillonite, are arranged in two silica sheets and one alumina sheet. In contrast, fibrous clay particles are made up of aggregates of fibers or ribbons, not sheets or platelets.
In PCT Patent Application WO 2006/069128, herein incorporated by reference in its entirety, a polyester nanocomposite is prepared via in situ polymerization of the polyester precursor(s) in the presence of exfoliated fibrous clay nanoparticles.
H.-C. zur Loye et al., in PCT Application WO2006012581, disclose a process for exfoliating particles into a polymer material wherein the step of combining synthetic oxide particles with a polymeric material comprises the steps of: exfoliating the particles into a monomer; and polymerizing the monomer to form a polymer composite material. Useful synthetic oxides include any synthetic oxides that have a plate-like shape with a thickness of less than about 5 nm and that are capable of being exfoliated into a polymer. Examples are synthetic hectorite and layered perovskites. During exfoliation, the object is to break many layers apart so as to form single layer particles or particles that have only a few layers, which are referred to as tactoids. In one embodiment, this is accomplished by applying shear forces to a solution of up to 5 wt % synthetic oxide particles in ethylene glycol and using the resulting suspension with additional monomers in a polyester polymerization process.
There remains a need for an improved process for preparing a polymer nanocomposite via in situ polymerization with improved dispersion of the nanofiller in order to enhance improvement in properties such as tensile strength and modulus.